This invention is related to flow control valves for mixing two fluids, and more specifically to such a valve in which a spool is moved to positions in response to changing inlet fluid pressures to maintain a constant ratio of the two fluids in the mixture.
An example of such an application includes mixing a solvent with compressed air.
A similar valve was disclosed in U.S. Pat. No. 4,924,900 which was issued May 15, 1990 to Frank A. Taube, Sr. and Edward J. Rozniecki in which a spool was mounted in a flow regulator in such a manner that two fluids could be mixed in a predetermined ratio and the ratio maintained regardless of variations in the incoming fluid pressures. The spool forms a variable orifice for passing one of the fluids, and maintains a stable position when the two fluid pressures are balanced at opposite ends of the spool. The orifice opening depends on the incoming fluid pressures. When one of the pressures is increased or reduced, the pressure imbalance moves the spool to a different balanced position, thereby enlarging or reducing the variable orifice opening so that the volumetric ratio of the two fluids in the mixture remains constant.
This invention represents an improvement over the ratio mixing valve disclosed in the '900 patent by providing a more compact device. In the preferred embodiment, both incoming fluids pass through the spool valve chamber. The spool has annular grooves for passing the two fluids through the chamber. One groove is sufficiently wide that regardless of the spool's position, the first fluid volumetric flow or pressure remains the same. The other groove is disposed so that the spool partially blocks the passage of the second fluid, forming a variable orifice opening that reduces the pressure of the second fluid to that of the first fluid.
The means for adjusting the volumetric ratio of the fluid mix comprise a pair of needle valves that are mounted in the two fluid passages downstream of the spool chamber, adjacent a pair of discharge ports. The user adjusts the ratio of the two fluids in the mixture by adjusting the position of the two needle valves.
Still another advantage of the invention is that the dither may be optimized. Dither is the tendency of a spool valve to oscillate axially in the valve chamber when the spool is balanced between two fluid pressures. If the dither or oscillation is too high, the spool tends to chatter thereby reducing its life. On the other hand, if the dither is too low, the spool tends to stick in the valve chamber thereby creating hysterisis and creating an error of inlet pressure and therefor a ratio error.
Although dither can occur in a liquid/liquid or a gas/gas system, dither may be most objectionable in a valve that mixes a gas with a liquid because the liquid is relatively incompressible while the gas is compressible.
I have found that the amount of dither can be optimized by controlling the diameter of the vent passages connecting the primary fluid passages to the ends of the spool valve chamber. Changing the vent hole size changes the time required for the spool to sense a fluid pressure change. Controlling the vent hole size, controls the sensing time, which controls the speed and duration of the oscillation. Controlling the vent hole size can maximize or eliminate oscillation. However, some oscillation is desirable because it creates a chopping action which is desirable when the output is used in a cleaning system, such as paint lines.
Initially the valve body is formed with vent passages of a certain diameter. Fluids are then passed through the valve. A flow meter is connected to the discharge or mixing passage to sense flow fluctuations of the mixture. The flow meter needle fluctuates in accordance with the oscillation of the spool.
If the dither is unacceptable, then at the least one of the vent passages is enlarged or reduced. The flow meter is used again to sense fluctuations in the flow of the mixture. When the flow fluctuations are acceptable then the degree of dither is accepted.
Another method is to introduce a needle valve into the vent passage to adjust the rate of sensing the pressure changes thereby controlling the amplitude of the dither or oscillation.
Still further objects and advantages of the invention will become readily apparent to those skilled in the art to which the invention pertains upon reference to the following detailed description.